In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for transferring, on a silicon substrate or the like, a fine pattern, which is required for forming an integrated circuit comprising such a fine pattern. However, the conventional exposure techniques using light exposure have been close to the limit of the conventional photolithography method while semiconductor devices have had finer patterns at an accelerated pace. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (wavelength: 193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a wavelength further shorter than the ArF laser, has been considered as being promising as the next generation of exposure technique using an exposure wavelength shorter than 45 nm. In this specification, the term “EUV light” means a ray having a wavelength in a soft X ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
It is impossible to use EUV light in conventional dioptric systems as in photolithography using visible light or ultraviolet light, since EUV light is apt to be absorbed by any substances and since the refractive index of the substances is close to 1 at the above wavelength. For this reason, a catoptric system, i.e., a combination of a reflective photomask and a mirror, is employed in EUV light lithography.
A mask blank is a stacked member to be used for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, formed thereon in this order. The reflective layer is usually a Mo/Si multilayer reflective film, which comprises silicon (Si) layers as high refractive layers and molybdenum (Mo) layers as low refractive layers alternately stacked to increase a light reflectance when the layer surface is irradiated with EUV light. Hereinafter, in this specification, the reflective layer comprising silicon (Si) layers as high refractive layers and molybdenum (Mo) layers as low refractive layers alternately stacked in a plurality of layers, will be referred to also as a Mo/Si multilayer reflective film.
The absorber layer is made of a material having a high absorption coefficient to EUV light, specifically, for example, a material containing chromium (Cr) or tantalum (Ta) as the main component.
Between the reflective layer and the absorber layer, usually a protective layer is formed. The protective layer is provided for the purpose of protecting the reflective layer so that the reflective layer will not be damaged by an etching process to be carried out for the purpose of forming a pattern on the absorber layer. Patent Document 1 proposes to use ruthenium (Ru) as the material of the protective layer. Patent Document 2 proposes a protective layer made of a ruthenium compound (Ru content of from 10 to 95 at %) containing Ru and at least one member selected from Mo, Nb, Zr, Y, B, Ti and La.